1. Technical Field
The present invention relates to an apparatus for regulating the ratio of the amount of fuel and the amount of air in a natural gas powered internal-combustion engine intended to work in a specified engine-specific load-rotation speed diagram. The apparatus further includes: a throttle for controlling the amount of air supplied to combustion chambers in the internal-combustion engine, injection devices for controlling the amount of natural gas supplied to the combustion chambers, and control devices for controlling the throttle and the injection devices. The control device is arranged to control the ratio of the amount of fuel and the amount of air dependent on the current operating point in the internal-combustion engine load-rotation speed diagram. The invention also relates to a method of regulating the ratio of the amounts of fuel and air in a natural gas powered internal-combustion engine utilizing such an apparatus.
2. Background
Natural gas engines are mainly used as alternatives to diesel engines because natural gas engines are lower in emissions than are diesel engines. In particular, particle production during combustion of the fuel in a diesel engine is a problem, and for which reason there have been attempts to reduce particle emissions using particle traps downstream of such diesel engines. This development direction, however, is associated with technical, as well as economic problems. Therefore, natural gas engines can be used as alternatives to diesel engines.
Today""s natural gas fueled internal-combustion engines designed for powering heavy vehicles are modified diesel engines. Internal-combustion engines are designed so that a maximum allowed operation temperature is not exceeded. If this temperature is exceeded, the thermal load may lead to engine breakdown. Such a breakdown can take place in the engine""s main components. An example would be that the pistons of an internal-combustion engine can become welded to the walls of the combustion chamber of an engine if temperatures are allowed to go too high. Alternatively, a breakdown can take place in the exhaust pipes of the vehicle downstream of the combustion chambers where, for instance, turbochargers can be subjected to thermal overload. To avoid these problems, present natural gas fueled internal-combustion engines are run with a fuel-air ratio of xcex=1.5.
This operation has shown itself to have the disadvantage that a catalyzer/catalyst/catalytic converter located downstream of the internal-combustion engine becomes poisoned because of the occurrence of sulphur in the fuel and engine oil.
During operation of heavy vehicles it is important that their response at low engine speeds be good, by which it is meant that available torque at and near idle-speed is large. It has been shown, however, that natural gas fueled vehicles in lean operation have relatively low torque at idle speed compared to the available torque at the operating point or speed of the engine that produces maximum power.
An objective of the invention is to provide a method and apparatus for controlling the ratio between the amount of fuel and the amount of air in a natural gas fueled internal-combustion engine where the operation of the internal-combustion engine allows regeneration of a catalyzer/catalyst located downstream of the internal-combustion engine. An associated objective is to provide an apparatus and a method for regulating the ratio of the amount of fuel and the amount of air in a natural gas fueled internal-combustion engine which allows an increase in the available torque at idle speed.
By dividing the internal-combustion engine""s load-rotation speed diagram into a first area where the internal-combustion engine is operated leanly and a second area where the internal-combustion engine is operated stochiometrically so that the area of stochiometric operation is arranged to be used at operation points in the load-rotation speed diagram with low load and/or low rotation rate, an internal-combustion engine is achieved for which the available torque at idle speed is substantially increased and for which regeneration of the catalyzer is possible through intermittent operation in uniform operation; that is, operation at the stochiometric ratio.
The same advantages are achieved when the internal-combustion engine is operated stochiometrically at operation points in the internal-combustion engine""s load-rotation rate diagram with low power and such that the internal-combustion engine operates leanly at operation points in the load-rotation rate diagram with high power.
According to a preferred embodiment of the invention, the specified operating point map of the engine for xcex value control in the vehicle""s load-rotation speed diagram is so arranged that the boarder line between lean operation and stochiometric operation is placed so that the temperature downstream of the internal-combustion engine""s combustion chamber exhaust ports is limited to a maximum allowed temperature. This temperature limit usually amounts to about 700xc2x0 C.
In a further preferred embodiment of the invention, the existing operating point in the internal-combustion engine""s load-rotation speed diagram is determined by using a parameter that is independent of which operational state the internal-combustion engine is in for the moment, such as the parameter required torque. By using a control parameter that is not affected by the internal-combustion engine""s momentary operational state, stable operation and simple control algorithms are obtained compared to when a dependent parameter such as load, rotation rate or developed torque is used.
In a further preferred embodiment, there is a step-by-step change in the xcex value at the transition from lean operation to stochiometric operation and at the transition from stochiometric operation to lean operation. In an especially preferred embodiment this stepwise change takes place in the form of a linear ramp. Through this procedure, the risk of abrupt changes occurring in the developed torque on changing operation areas with internal-combustion engines is reduced.
In yet another preferred embodiment of the invention, the first and second areas partly overlap each other whereby unstable changes between the first area and the second area are suppressed.